The research deals with the structure and functions of leukocytes in various types of reactions. Studies are continuing on intracellular mechanisms that permit certain parasites to survive and multiply within macrophages, and on modifications in these mechanisms in cells from the immunized animal. Detailed observations are being made at the subcellular morphologic and biochemical level on the mechanisms involved in incorporation in macrophages of iron into ferritin, and in storage and mobilization of ferritin in these cells. Megakaryocytes have been isolated from guinea pig bone marrow. We are attempting to obtain information on morphology of the maturation of these cells, and on the mechanisms that underlie platelet formation. Studies are to be made on eosinophils, comparing their locomotion and chemotactic responses to those of neutrophils, in the hope of obtaining clues to eosinophil function.